Controlling solids in electrodepositable compositions

ABSTRACT

This invention relates to a method of controlling electrodeposition bath solids. More particularly, this is a method by which the concentration of solids in an electrodeposition bath is maintained at a desired level by utilizing a membrane-controlled selective separation process such as an ultrafiltration process.

[ 51 May 16, 1972 United States Patent Christenson et a].

[56] References Cited UNITED STATES PATENTS [54] CONTROLLING SOLIDS IN ELECTRODEPOSITABLE COMPOSITIONS l m m t. e um W3 6 re BL 0 7 9 l W O 9 M 3 [72] Inventors: Roger M. Christenson; Louis R. Le Bras,

both of Gibsonia. Pa.

Wallace et a1.

[73 I Assigncc: II'G Industrles, Int, Pittsburgh, Pa. [22] Filed: Sept. 23, 1970 FOREIGN PATENTS OR APPLICATIONS 1,071,458 6/1967 Great Britain....

Primary Examiner-Howard S. Williams AttorneyChishoIm and Spencer [21] Appl. No.:

[ ABSTRACT This invention relates to a method of controlli Related US. Application D818 Continuation-impart of Ser. No. 81

n electrodeposition bath solids. More particularly, this is a r nethod by which the concentration of solids in an electrodeposition bath is maintained at a desired level by utilizing a membranecontrolled selective separation process such as an ultrafiltration process.

101 9 808 y r I 4 M 0 .32 0 m2" H 1 m 4 "m "5 1 I l 0 B 1969, abandoned.

[51] Int. [58] Field 11 Claims, 2 Drawing Figures Patented May 16, 1972 INVENTOR; Kama M. CI/fi/SEMQW Jews 9. 1: K45

CONTROLLING SOLIDS IN ELECTRODEPOSITABLE COMPOSITIONS Cross-Reference to Related Applications This application is a continuation-in-part of application Ser. No. 814,789, filed Apr. 9, 1969 now abandoned.

STATE OF THE ART In recent years electrodeposition has become a widely commercially-accepted coating technique. Coatings deposited electrically have excellent properties for many applications.

Films applied by electrodeposition do not run, tear, drip, sag, or wash off during baking. Virtually any electrically conductive surface may be coated by electrodeposition. Treated and untreated metals such as iron, steel, copper, zinc, brass, tin, nickel, chromium and aluminum are among the most commonly employed substrates. However, other substrates, such as paper impregnated with substances to render it conductive under conditions of the coating process, may also be employed.

Such electrodeposition processes are carried out in various ways. For example, in many cases the articles to be coated, usually after a pretreatment step, are immersed in an aqueous dispersion of a solubilized, ionized, film-forming material such as a synthetic organic vehicle resin. An electric current is passed between the article to be coated, serving as an electrode, and a counter-electrode to cause electrodeposition of a coating of the vehicle resin on the article. The article is then withdrawn from the bath, usually rinsed, and then the coating is either air-dried or baked in the manner of a conventional film.

In the electrodeposition process, as the articles are continually coated, the solids content of the electrocoating bath, i.e., resin, pigments, etc., are depleted, necessitating the replenishment of the bath solids. In the art, there are three commonly utilized methods of replenishing the depleted bath solids. One method is by employing an in-line blender to blend unsolubilized vehicle with the bath material. By utilizing the bath material, the solubilizing agent which was freed during the electrocoating process is used to aid in the partial solubilization of unsolubilized vehicle which is to be incorporated into the bath. Normally, the ratio of bath material to the unsolubilized resin is high, so as to insure sufficient solubilizing agent in order to facilitate adequate solubilization before incorporation into the bath. Generally the pigment paste in this process is not incorporated via the in-line blender, although it can be if desired. Ordinarily, the pigment paste is added to a premix container and reduced to pumping solids with bath material.

Another method of replacing the depleted solids is by adding a fully solubilized replenishing feed and controlling the system by employing membranes or ion exchanging.

Perhaps the most common method of introducing replenishment material into the bath is by utilizing a feed stock that is deficient in solubilizing agent. The amount of solubilizing agent employed is sufficient to produce the desired colloidal dispersion, emulsion or solution when reduced to coating solids. In contrast with the in-line blending process, the replenishment which is deficient in solubilizing agent is premixed at low shear with a small amount of bath material which is utilized as a reducing medium to facilitate ease of pumping.

However, utilizing a replenishing feed that is deficient in solubilizing agent is not entirely trouble free. For example, such feeds contain all the components necessary to produce a coating composition and should be formulated at relatively low solids content in order to permit adequate solubilizations; however, this in practice presents problems because continued addition of the low solids replenishment to the bath to replace the depleted solids tends to result in overflow of the bath.

Also, in the absence of facilities for drying the article after pretreatment, water from any pretreatment step is carried into the bath and causes further dilution of the bath solids. Partial or complete spray rinsing of the electrocoated articles over the electrocoating bath, if used, will also dilute the bath. In other instances, leaking valves have introduced undesirable quantities of water into the bath.

Ultrafiltration of an aqueous medium containing electrodepositable solids has been described in copending application Ser. No. 883,584, filed Dec. 9, 1969, and now abandoned, wherein the ultrafiltration process is taught to be a means of reducing dragout loss by rinsing the dragout off the article electrodeposited with deionized water over the electrocoating bath and then subjecting at least a portion of the electrodeposition bath to ultrafiltration, thereby controlling the composition of the bath. Also, ultrafiltration of an aqueous medium containing electrodepositable solids has been described in copending application Ser. No. 814,789, wherein ultrafiltration process is taught to be a means of controlling bath composition by removal of objectionable accumulated materials through use of a selective filtration process, that is, a process of selectively removing low molecular weight materials from the electrodeposition bath.

DESCRIPTION OF THE INVENTION It has now been discovered that by subjecting at least a portion of the electrocoating bath to either a continuous or intermittent selective filtration process such as ultrafiltration, the solids content of the bath can be maintained at a desired level and that water and low molecular size material can be removed from the bath composition. By the removal of the excess water, the bath solids parameters are maintained and overflow is prevented. With properly selected membranes, this selective filtration process does not remove any product or desirable resin, pigment, or the like, from the electrocoating bath composition, but does permit the passage of water and low molecular weight solute such as amines, alkaline metal ions, phosphates, chromates, sulfates, solvents and dissolved carbon dioxide, and the like, when desirable.

The filtrate recovered from this selective separation process has been utilized in a number of different ways, for example, as a rinsing agent, see copending applications Ser. No. 881,259, filed Dec. 1, 1969, now abandoned and Ser. No. 879,769, filed Nov. 25, 1969, now abandoned wherein it is taught that ultrafiltrate may be contacted with an ion exchange resin, thereby producing water of high purity. In some instances, the ultrafiltrate may be disposed of by drain or utilized in other areas such as pretreatment cleaning baths or in spray cleaning compositions and may also be utilized in treatment and spray solution compositions as a solvent.

The concentrate obtained by the selective separation process is ordinarily returned to the electrodeposition bath as necessary to maintain the desired solids content of the electrodeposition bath.

One of the preferred areas of use of this invention is where the replenishment material added to the bath has relatively low solids content, for example, from about 12 percent solids content is used to feed an electrodeposition bath. In many desirable processes, the solids content of the feed material ranges from about 20 percent to about 35 percent and the present invention is particularly useful in such processes.

The utilization of ultrafiltration in such instances provides for bath solids content control by the removal of water.

Another area of preferred use of this invention is to remove water that has been carried into the electrodeposition bath on and trapped in articles of complex shapes and structures that are to electrocoated. Ordinarily, articles to be coated are, prior to such coating, degreased or pretreated with an aqueous material, such as a phosphate solution or the like. After such preparatory treatments, the articles may also be rinsed with water and, in some instances, the parts enter the bath without adequate drying, so that they remain wet. Even when the articles are dried, as by passing the wet articles through a dehydration oven, they generally still enter the bath containing trapped pockets of water, especially if, as is usually the case, the article is of complex shape.

In such cases, the use of ultrafiltration as described herein is a preferred method of removing the excess water buildup in the bath.

Ultrafiltration may be defined as a method of concentrating solute while removing solvent, or selectively removing solvent and low-molecular weight solute from a significantly higher molecular weight solute. From another aspect, it is a process of separation whereby a solution containing a solute of molecular dimensions significantly greater than the solvent is depleted of solute by being forced under a hydraulic pressure gradient to flow through a suitable membrane. The first definition is the one which most fittingly describes the term ulrafiltration as applied to an electrodeposition bath.

Ultrafiltration thus encompasses all membrane-moderated, pressure-activated separations wherein solvent or solvent and smaller molecules are separated from modest molecular weight macromolecules and colloids. The term ultrafiltration" is generally broadly limited to describing separations involving solutes of molecular dimensions greater than about ten solvent molecular diameters and below the limit of resolution of the optical microscope, that is, about 0.5 micron. ln the present process, water is considered the solvent.

The principles of ultrafiltration and filters are discussed in a chapter entitled Ultrafiltration" in the Spring, 1968, volume of ADVANCES IN SEPARATIONS AND PURIFICATIONS, E. S. Perry, Editor, John Wiley & Sons, New York, as well as in CHEMICAL ENGINEERING PROGRESS, Vol. 64, Dec. 1968, pages 31 through 43, which are hereby incorporated by reference.

The basic ultrafiltration process is relatively simple. Solution to be ultrafiltered is confined under pressure, utilizing, for example, either a compressed gas or liquid pump in a cell, in contact with an appropriate filtration membrane supported on a porous support. Any membrane or filter having chemical integrity to the system being separated and having the desired separation characteristics may be employed. Preferably, the contents of the cell should be subjected to at least moderate agitation to avoid accumulation of the retained solute on the membrane surface with the attendant binding of the membrane. Ultrafiltrate is continually produced and collected until the retained solute concentration in the cell solution reaches the desired level, or the desired amount of solvent or solvent plus dissolved low molecular weight solute is removed. A suitable apparatus for conducting ultrafiltration is described in U. S. Pat. No. 3,495,465, which is hereby incorporated by reference.

There are two types of ultrafiltration membranes. One is the microporous ultrafilter, which is a filter in the traditional sense, that is, a rigid, highly voided structure containing interconnected random pores of extremely small average size. Through such a structure, solvent (in the case of electrodeposition, water) flows essentially viscously under a hydraulic pressure gradient, the flow rate proportional to the pressure difference, dissolved solutes, to the extent that their hydrated molecular dimensions are smaller than the smallest pores within the structure, will pass through, little impeded by the matrix. Larger size molecules, on the other hand, will become trapped therein or upon the external surface of the membrane and will thereby be retained. Since the microporous ultrafilters are inherently susceptible to internal plugging or fouling by solute molecules whose dimensions lie within the pore size distribution of the filter, it is preferred to employ for a specific solute a microporous ultrafilter whose mean pore size is significantly smaller than the dimensions of the solute particle being retained.

In contrast, the diffusive ultrafilter is a gel membrane through which both solvent and solutes are transported by molecular diffusion under the action of a concentration or activity gradient. In such a structure, solute and solvent migration occurs via random thermal movements of molecules within and between the chain segments comprising the polymer network, Membranes prepared from highly hydrophilic polymers which swell to eliminate standard water are the most useful diffusive aqueous ultrafiltration membrane.

Since a difi'usive ultrafilter contains no pores in the conventional sense and since concentration within the membrane of any solute retained by the membrane is low and time-independent, such a filter is not plugged by retained solute, that is, there is no decline in solvent permeability with time at a constant pressure. This property is particularly important for a continuous concentration or separation operation. Both types of filters are known in the art.

The presently preferred ultrafilter is an anisotropic membrane structure such as illustrated in FIG. 1.

This structure consists of an extremely thin, about one-tenth to about 10 micron layer, of a homogeneous polymer 1 supported upon a thicker layer of a microporous open-celled sponge 2, that is, a layer of about 20 microns to about 1 millimeter, although this dimension is not critical. If desired, this membrane can be further supported by a fibrous sheet, for example, paper, to provide greater strength and durability. These membranes are used with a thin film or skin side exposed to the high pressure solution. The support provided to the skin by the spongy substrate is adequate to prevent film rupture.

Membranes useful in the process are items of commerce and can be obtained by several methods. One general method is described in Belgian Pat. No. 721,058. This patent describes a process which, in summary, comprises (a) forming a casting dope of the polymer in an organic solvent, (b) forming a film of the casting dope, and (c) preferentially contacting one said of said film with a diluent having high compatibility with the casting dope to effect precipitation of the polymer immediately upon coating the cast film with the dope to effect precipitation of the polymer immediately upon coating the cast film with the diluent.

The choice of a specific chemical composition for the membrane is determined to a large extent by its resistance to the chemical environment. Membranes can be typically prepared from thermoplastic polymers such as polyvinyl chloride, polyacrylonitrile, polysulfones, poly(methyl methacrylate), polycarbonates, poly(n-butyl methacrylate), as well as a large group of copolymers formed from any of the monomeric units of the above polymers, including Polymer 360, a polysulfone copolymer. Cellulosic materials such as cellulose acetate may also be employed as membrane polymers.

Some examples of specific anisotropic membranes operable in the process of the invention include Diaflow membrane ultrafilter PM-30, the membrane chemical composition of which is a polysulfone copolymer, Polymer 360, and which has the following permeability characteristics:

Flow Rate ml./min.

0.25% Pepsin Membrane Pressure Distilled In Distilled Diameter P.S.I. Water Water (55 psi) 25 mm. 55 8.6 1.1 I50 mm. 55 350.0 46.0

The membrane is chemically-resistant to acids (l-ICl, H l-l PO all concentrates), alkalis, high phosphate buffer and Flux Molecular Percent (gal./sq.ft./day at Solute Weight Retention 30 psi, 1.0% solute Cytochrome C 12,600 50 100 aChymotripsinagen 24,000 90 22 Ovalbumin 45,000 100 45 This membrane is hereinafter referred to as Membrane B."

Dorr-Oliver BPA type membrane, the membrane chemical composition of which is phenoxy resin (polyhydroxy ether), and which has the following permeability characteristics:

Flux Molecular Percent (gal.lsq.ft./day at Solute Weight Retention 30 psi, 1.0% solute Cytochrome C 12,600 50 30 This membrane is hereinafter referred to as Membrane C."

The microporous ultrafilters are generally isotropic structures, thus flow and retention properties are independent of flow direction. It is preferred to use an ultrafilter which is anisotropic in its microporous membrane structure, FIG. 2. In such a membrane, the pore size increases rapidly from one face to the other. When the fine-textured side 4 is used in contact with the feed solution, this filter is less susceptible to plugging since a particle which penetrates the topmost layer cannot become trapped in the membrane because of the larger pore size 5 in the substrate.

The process of the invention may be operated as either a batch or a continuous process. In batch selective filtration or batch ultrafiltration a finite amount of material is placed in a cell which is pressurized. A solvent and lower molecular weight solutes are passed through the membrane. Agitation is provided by a stirrer, for example, a magnetic stirrer. Obviously, this system is best used for small batches of material. In a process requiring continuous separation, a continuous selective filtration process is preferred. Using this technique, material is continuously recirculated under pressure against a membrane or series of membranes through interconnecting flow channels, for example, spiral flow channels.

Likewise, the ultrafiltration process may be conducted as either a concentration process or a diafiltration process. Concentration involves removing solvent and low molecular weight solute from an increasingly concentrated retentate. Filtration flow rate will decrease as the viscosity of the concentrate increases. Diafiltration, on the other hand, is a constant volume process whereby the starting material is connected to a reservoir of pure solvent, both of which are placed under pressure simultaneously. Once filtration begins, the pressure source is shut off in the filtration cell and thus, as the filtrate is removed, an equal volume of new solvent is introduced into the filtration cell to maintain the pressure balance. The configuration of the filter may also vary widely and is not limiting to the operation of the process. The filter or membrane may, for example, be in the form of a sheet, tubes, or hollow fiber bundles, among other configura-tions.

Under ideal conditions, selected low molecular weight solutes would be filtered as readily as solvent and their concentration in the filtrate is equal to that in the retentate. Thus, for example, if a material is concentrated to equal volumes of filtrate and retentate, the concentration of low molecular weight solute in each would be the same.

Using diafiltration, retentate solute concentration is not constant and the mathematical relationship is as follows:

lo/ l) VIIVO where C is the initial solute concentration, C, is the final solute concentration of the retentate, V, is the volume of solute delivered to the cell (or the volume of the filtrate collected), and V, is the initial solution volume (which remains constant).

Electrodepositable compositions, while referred to as solubilized, in fact are considered a complex solution, dispersion or suspension or combination of one or more of these classes in water, which acts as an electrolyte under the influence of an electric current. While, no doubt, in some circumstances the vehicle resin is in solution, it is clear that in some instances and perhaps in most the vehicle resin is a dispersion which may be called a molecular dispersion of molecular size between a colloidal suspension and a true solution.

The typical industrial electrodepositable composition also contains pigments, crosslinking resins and other adjuvants which are frequently combined with the vehicle resin in a chemical and a physical relationship. For example, the pigments are usually ground in a resin medium and are thus wetted with the vehicle resin. As can be readily appreciated then, an electrodepositable composition is complex in terms of the freedom or availability with respect to removal of a component or in terms of the apparent molecular size of a given vehicle component.

As applied to the process of this invention, ultrafiltration comprises subjecting an electrodepositable composition, especially after it has been employed in a coating process or aged, which inherently causes contaminants and other low molecular weight materials to accumulate in the bath, such as metal pretreatment chemicals, water, absorbed CO, (either dissolved or, more likely, combined as an aminicsalt or carbonate), neutralizing agent, organic solvent and ions such as formate, chromate, phosphate, chloride and sulfate, for example, to an ultrafiltration process employing an ultrafilter, preferably a diffusive membrane ultrafilter selected to retain the solubilized vehicle resin while passing water and low molecular weight solute, especially those with a molecular weight below about 500. As previously indicated, the filters discriminate as to molecular size rather than actual molecular weight, thus, these molecule weights merely establish an order of magnitude rather than a distinct molecular weight cut-off. Likewise, as previously indicated, the retained solutes may, in fact, be colloidal dispersions or molecular dispersions rather than true solutes.

In practice, a portion of the electrodepositable composition may be continuously or intermittently removed from the electrodeposition bath and passed under pressure created by a pressurized gas or by means of pressure applied to the contained fluid in contact with the ultrafilter. Obviously, if desired, the egress side of the filter may be maintained at a reduced pressure to create the pressure difference.

The pressures necessary are not severe. The maximum pressure, in part, depends on the strength of the filter. The minimum pressure is that pressure required to force water and low molecular weight solute through the filter at a measurable rate. With the presently preferred membranes, the operating pressures are between about 10 and p.s.i., preferably between about 25 and 75 p.s.i. Under most circumstances, the ultrafilter should have an initial flux rate, measured with the composition to be treated of at least about 3 gals./sq.ft./day (24 hours) and preferably at least about 4.5 gal./sq.ft./day.

As previously indicated, the bath composition should be in motion at the face of the filter to prevent the retained solute from impeding the flow through the filter. This may be accomplished by mechanized stirring or by fluid flow with a force vector to the filter surface.

The retained solutes comprising the vehicle resin are then returned to the electrodeposition bath. If desired, the concentrate may be reconstituted by the addition of water either before entry to the bath or by adding water directly to the bath.

If there is present in the bath desirable materials which, because of their molecular size, are removed in the ultrafiltration process, these may likewise be returned to the bath either directly to the retained solute before entry to the bath, in the makeup feed as required, or independently.

A number of electrodepositable resins are known and can be employed to provide the electrodepositable compositions which may be utilized within the scope of ultrafiltration. Virtually any water-soluble, water-dispersible or water-emulsifiable vehicle resin in an aqueous medium can be electrodeposited and, if film-forming, provides coatings which may be suitable for certain purposes. The present invention is applicable to any such process.

Presently, the most widely used electrodeposition vehicle resins are synthetic polycarboxylic resinuous materials. Numerous such resins are described in U. S. Pat. Nos. 3,230,162; 3,441,489; 3,422,044, 3,403,088; 3,369,983; 3,366,563; 3,516,913 and 3,518,212. They include alkyd resins; modified or unmodified adducts of drying oil or semi-drying oil fatty esters with a dicarboxylic acid or anhydride, such as maleic anhydride adducts of linseed oil, soybean oil, or the like, modified in some cases with monomers such as styrene or a polyol; acrylic polymers, such as acid-containing interpolymers of acrylic monomers, in many cases including a hydroxyalkyl ester; mixed partial esters of fatty acids with resinous polyols, such as polyols derived from epoxy resins or styrene-ally] alcohol copolymers; and others, including certain phenolic resins, hydrocarbon resins, etc. Aminoplast resins, usually made from condensation of melamine, urea, benzoguanamine or the like with formaldehyde and etherified with an alcohol such as methanol, butanol, hexanol or a mixture of alcohols, are also useful, especially in combination with hydroxyl-containing alkyd or acrylic resins.

In order to produce an electrodepositable composition from such polycarboxylic acid resins, it is necessary to at least partially neutralize the acid groups present with a base in order to disperse the resin in the aqueous electrodeposition bath. Inorganic bases such as metal hydroxides, especially potassium hydroxide, can be used, as can ammonia or organic bases such as amines. Water-soluble amines are often preferred. Commonly used amines include ethylamine, diethylamine, triethylamine, diethanolamine, and the like.

Other base-solubilized polyacids which may be employed as electrodeposition vehicles include those taught in U. S. Pat. No. 3,392,165, which is incorporated herein by reference, wherein the acid groups rather than being solely polycarboxylic acid groups contain mineral acid groups such as phosphonic, sulfonic, sulfate and phosphate groups.

The process of the instant invention is equally applicable to cationic type vehicle resins, that is, vehicle resins which deposit on the cathode. These include polybases solubilized by means of an acid, for example, an amine-terminated polyamide or an acrylic polymer solubilized with acetic acid. Other cationic polymers include reaction products of polyepoxides with amino-substituted boron esters and reaction products of polyepoxides with hydroxyl or carboxyl-containing amines; many such products are described in copending application Ser. Nos. 772,353 and 772,366, (now abandoned) both filed Oct. 28, 1968, and Ser. Nos. 840,847 and 840,848, both filed July 10, 1969 and both now abandoned.

In addition to the vehicle resin, there may be present in the electrodepositable composition any desired pigment or pigment composition, including practically any of the conventional types of pigments employed in the art. There is often incorporated into the pigment composition a dispersing or surface active agent. Usually the pigment and surface-active agent, if any, are ground together in a portion of the vehicle, or alone in an aqueous medium, to make a paste and this is blended with the vehicle to produce a coating composition.

In many instances, it is preferred to add to the electrodeposition bath certain additives to aid dispersibility, viscosity and/or film quality, such as a non-ionic modifier or solvent. There may also be included additives such as antioxi- Example I The vehicle resin in this example is a maleinized tall oil fatty acid-adipic acid of a styrene-allyl alcohol copolymer of 1 100 molecular weight and 5 hydroxyl functionality comprising 39.8 percent of the copolymer, 52.9 percent tall oil fatty acids, 1.3 percent adipic acid and 6.0 percent maleic anhydride as an percent solution in 4-methoxy-4-methyl pentanone-2, having an intrinsic viscosity of 4,000 centipoises and an acid number of 40. Also present is a small amount of linseed oil reacted with 20 percent by weight of maleic anhydride. The electrodeposition bath had the following composition:

Parts by Weight Vehicle resin above at percent solids 200.0 20% maleinized linseed oil 7.8

Dispersing agent (combination of oilsoluble sulfonate and non-ionic surfactant Witco 912) 4-methoxy-4-methyl-pentanone-Z Cresylic acid Diethylamine Potassium hydroxide Deionized water Red iron oxide Anthracite coal (pigmentary) Amine-modified clay Strontium chromate Basic lead silicate Manganese dioxide Two thousand parts of the above bath at 12 percent solids were subjected to a semi-continuous electrodeposition process as follows: Approximately 5 percent of the resin solids were coated on aluminum coil stock at 200 volts for 5 minutes with the bath temperature 7580 F. In order to reconstitute the bath solids, it is necessary to add 12.0 parts of feed stock solids which may be formulated at a wide range of solid concentrations, which is somewhat dependent on the particular resin viscosity employed and on the solubilizing equipment utilized. In this instance, the percent solids of the feed stock was 43.8 and the analytical tests indicated that 11.8 parts of bath solids had been depleted from the coating composition, which would require a feed stock addition of 27.0 parts. However, in the depletion of the l 1.8 parts of solids from the bath, only an additional 12.2 parts of water were removed from the bath, either by dragout or evaporation or a combination of both, as a result there is only room for 24.0 parts of feed stock, which will, in fact, only introduce 10.5 parts of feed stock solids into the path. Therefore, it is necessary to subject a portion of the bath to ultrafiltration and selectively separate at least 3.0 parts of efiluent to permit the introduction of 3.0 parts of feed stock to return the bath to the proper operating solids.

This was carried out by a selective separation utilizing an Amicon cylindrical ultrafilter (Model 2000) and a Diaflow Membrane Ultrafilter PM-3O (supra), which while subjected to a pressure of 50 p.s.i. maintained a flow rate from about 3 to 5 milliliters per minute. The Diaflow Membrane Ultrafilter PM-30 (Membrane A) had the characteristics as hereinabove described.

The resin composition in this example was a reaction product of 20 percent maleic anhydride and 80 percent linseed oil having a viscosity of 100,000 centipoises at 25 C.

The electrodeposition bath had the following composition:

Vehicle non-volatiles: 92.6%

Maleinized oil 96.7%

Cresylic acid 2.88%

Dispersing agent (combination of oilsoluble sulfonate and non-ionic surfactant Witco 912) 0.37%

Pigment: 7.4% Carbon black 75.5%

Strontium chromate 24.5%

Amine:

Diethylamine 12% based on total resin solids The electrodeposition bath was charged at 8 percent solids and contained 1 pound/100 gallons of 37 percent formalin.

A 2000 part bath was operated until 25 percent of the bath solids were depleted. A 36.4 percent feed stock was employed to reconstitute the bath solids. During depletion of the bath solids the total bath composition was also depleted by 90 parts, some was produced by dragout and some probably from evaporation of water for the bath. In order to restore the bath solids to 8 percent, it would necessitate adding llO parts of feed stock which would be 20 parts in excess of the baths capacity. A portion of the bath was ultrafiltered whereby at least 20 parts of ultra-filtrate were removed from the system. This enabled the introduction of the entire 110 parts of feed stock into the bath, whereby the optimum bath solids could be maintained. The ultrafilter membrane employed in this example was the same as one that was employed in example I and the ultrafiltration was carried out at 50 psi.

EXAMPLE Ill The vehicle resin in this example is an Epon 1004- tall oil fatty acid mixed partial ester with a maleinized tall oil fatty acid adduct comprising 45 percent Epon 1004, 48 percent tall oil fatty acid and 7 percent maleic anhydride, having an acid value of 59 and a viscosity of 230,000 centipoises at 85 percent solids in butyl Cellosolve. The electrodepositable materi- Montmorillonite clay, modified with trimethyl octyl ammonium ions and containing 0.65% nitrogen (Bentone l 1) Potassium hydroxide Butyl Cellosolve Diethylamine Crcsylic acid Deionized water 1715.4

in order to conserve on paint utilization, the dragout was rinsed off the electrocoated article over the electrodeposition bath, whereby the bath captured both the dragout and the rinse water. By dragout" is meant that portion of the bath material that is not electrocoated to the article, but merely adherent thereto, or pocketed by complex-shaped articles, and is withdrawn from the bath. This dragout represents wasted material for, generally, it is not rinsed off over the tank, but rather in a separate section and is disposed of by drain. How ever, rinse water containing such dragout presents a disposal problem. Rinsing over the bath conserves bath solids and eliminates dragout disposal problems.

However, on continual rinsing over the bath, the bath solids were diluted and volume of the bath composition increased.

When the above composition was employed in capturing the rinse water containing the dragout, it was necessary to subject the bath to ultrafiltration to maintain optimum coating solids. The volume of ultrafiltrate selectively separated from the bath composition is dependent in part on the volume of rinsing water utilized in rinsing a particular article; volume of water carried in on the particular article to be coated; the rate of evaporation over the bath surface, the solids content of the feed stock; and the rate of depletion of solids due to the electrocoating process. In controlling the above variables in this composition, a Diaflow Membrane Ultrafilter PM-30, as previously described, was employed at 50 p.s.i.

In addition, in some instances when complex articles are pretreated, that is, contacted with aqueous treating solutions or aqueous rinsing solutions, before entering the electrocoating bath, the said articles enter the bath at least partially wet and containing trapped rinse water in sufficient quantity to cause diminution of the solids content, thereby causing undesirable effects on the coating parameters and film properties. In such cases, ultrafiltration may be employed successfully, utilizing the membranes A, B and C as herein described, to reduce the excess water build-up and to return the bath to stable operating conditions.

Other electrodepositable compositions, such as those hereinabove described, can be substituted for those of the examples. Likewise, various ultrafilters and method variations may be employed to obtain the improvement hereinabove described.

As shown, the process of the invention has many advantages over prior methods of controlling bath solids, for example, evaporation, and feeding high solids feed stock. Vaporization is difficult to control and undependable; likewise, the particular vehicle may not be amenable to formulation as high solids feed. For example, where a high solids feed which is deficient in solubilizing agent is necessary, considerable difficulties have been experienced in formulating stable compositions. However, by employing ultrafiltration, such compositions may be formulated at a solids content which provides for a more stable system.

According to the provisions of the patent statutes, there are described above the invention and what are now considered its best embodiments; however, within the scope of the appended claims, it is to be understood that the invention can be practiced otherwise than as specifically described.

We claim:

1. In a method of operating an electrodeposition process in which an article is electrocoated from an electrodeposition bath comprising solubilized ionized synthetic organic vehicle resin in an aqueous medium, wherein the bath has been depleted of vehicle resin, the steps comprising:

a. adding to said bath replenishment material comprising vehicle resin in aqueous medium, said replenishment material having a total solids content of less than about 50 percent by weight;

b. subjecting at least a portion of said electrodeposition bath to an ultrafiltration process wherein an ultrafiltration membrane retains the vehicle resin while passing water and solute of substantially lower molecular size than the vehicle resin, whereby excess water introduced by said replenishment material is removed; and

c. subsequently electrocoating an article.

2. A method as in claim 1 wherein the replenishment material is deficient in solubilizing agent.

3. A method as in claim 1 wherein the ultrafiltration process is operated at a pressure gradient between about and about 150 p.s.i. and the membrane utilized has a flux rate of at least about 4.5 gallons per square foot per day.

4. A method as in claim 3 wherein the resin is a base-solubilized synthetic polycarboxylic acid resin.

5. A method as in claim 4 wherein the base is potassium hydroxide.

6. A method as in claim 1 wherein the total solids content of the replenishment material is between about 12 and about 50 percent by weight.

7. A method as in claim 6 wherein the replenishment material is deficient in solubilizing agent.

8. A method as in claim 6 wherein the ultrafiltration process is operated at a pressure gradient between about 10 and about 150 p.s.i. and the membrane utilized has a flux rate of at least about 4.5 gallons per square foot per day.

9. A method as in claim 8 wherein the resin is a base-solubilized synthetic polycarboxylic acid resin.

10. A method as in claim 9 wherein the base is potassium hydroxide.

1]. ln a method of operating an electrodeposition coating process in which an article is electrocoated from an electrodeposition bath comprising solubilized ionized synthetic organic vehicle resin in an aqueous medium, the steps comprismg:

a. contacting the article to be coated with aqueous material prior to electrocoating;

b. electrocoating said articles while at least partially wet;

c. subjecting at least a portion of said electrodeposition bath to an ultrafiltrarion process whereby excess water derived from said aqueous material is removed; and

d. subsequently electrocoating an article.

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2. A method as in claim 1 wherein the replenishment material is deficient in solubilizing agent.
 3. A method as in claim 1 wherein the ultrafiltration process is operated at a pressure gradient between about 10 and about 150 p.s.i. and the membrane utilized has a flux rate of at least about 4.5 gallons per square foot per day.
 4. A method as in claim 3 wherein the resin is a base-solubilized synthetic polycarboxylic acid resin.
 5. A method as in claim 4 wherein the base is potassium hydroxide.
 6. A method as in claim 1 wherein the total solids content of the replenishment material is between about 12 and about 50 percent by weight.
 7. A method as in claim 6 wherein the replenishment material is deficient in solubilizing agent.
 8. A method as in claim 6 wherein the ultrafiltration process is operated at a pressure gradient between about 10 and about 150 p.s.i. and the membrane utilized has a flux rate of at least about 4.5 gallons per square foot per day.
 9. A method as in claim 8 wherein the resin is a base-solubilized synthetic polycarboxylic acid resin.
 10. A method as in claim 9 wherein the base is potassium hydroxide.
 11. In a method of operating an electrodeposition coating process in which an article is electrocoated from an electrodeposition bath comprising solubilized ionized synthetic organic vehicle resin in an aqueous medium, the steps comprising: a. contacting the article to be coated with aqueous material prior to electrocoating; b. electrocoating said articles while at least partially wet; c. subjecting at least a portion of said electrodeposition bath to an ultrafiltration process whereby excess water derived from said aqueous material is removed; and d. subsequently electrocoating an article. 